1. Technical Field
The present invention relates to a liquid crystal display panel of a fringe field switching (FFS) mode that includes a pair of substrates holding a liquid crystal layer, a common electrode provided to one of the substrates, and a pixel electrode provided corresponding to the common electrode with an insulator therebetween, and drives the liquid crystal layer by an electric field formed between the common electrode and pixel electrode. More particularly, the invention relates to a liquid crystal display panel that has slits whose ends are open on one side in each pixel electrode and provides bright displays without color mixture with improved alignment of a pixel electrode and a color filter.
2. Related Art
Over recent years, liquid crystal display panels have been much employed not only in information and telecommunications equipment but in electrical equipment in general. The liquid crystal display panels that have long been in use are made up of a pair of substrates of glass or the like with electrodes and so on formed on their surfaces, and a liquid crystal layer formed between such pair of substrates. Images of various types are displayed by the application of voltage to the electrodes on the two substrates, which rearranges the liquid crystals, altering the transmittance of light therethrough. This is what may be called the “longitudinal field mode”. Among such longitudinal field mode liquid crystal display panels there exist those with a twisted nematic (TM) mode or vertical alignment (VA) mode, which however have the problem that their viewing angle is narrow. Accordingly, longitudinal field mode liquid crystal display panels with various improvements such as a multidomain vertical alignment (MVA) mode have been developed.
On the other hand, liquid crystal display panels that may be called “transverse field mode” are also known as in-plane switching (IPS) mode liquid crystal display panels that differ from the longitudinal field mode described above in having electrodes on one substrate only (see JP-A-10-319371 and JP-A-2002-131767). The operating principles of such an IPS mode liquid crystal display panel will now be described using FIGS. 8 and 9. FIG. 8 is a schematic plan view of a single pixel portion of the IPS mode liquid crystal display panel. FIG. 9 is a cross-sectional view along line IX-IX in FIG. 8.
This IPS mode liquid crystal display panel 50 has an array substrate AR and a color filter substrate CF. The array substrate AR has a plurality of scan lines 52 and common wires 53 provided in parallel with one another on a surface of a first transparent substrate 51, and a plurality of signal lines 54 provided in the direction orthogonal to these scan lines 52 and common wires 53. In the central portion of each pixel there is provided a common electrode 55 having for example a comb-like shape as in FIG. 8 and extending strip-like from the common wire 53. A pixel electrode 56, likewise of a comb-like shape, is provided so as to enclose the spaces around the peripheries of the common electrode 55, and the surface of the pixel electrode 56 is covered with a protective insulator 57 of silicon nitride and an alignment layer 58 of polyimide, for example.
Close to the intersections of the scan lines 52 and signal lines 54 there are formed thin film transistors (TFTs) that serve as switching elements. For each TFT, a semiconductor layer 59 is laid between a scan line 52 and signal line 54; a signal line portion on the semiconductor layer 59 constitutes the TFT's source electrode S and a scan line portion below the semiconductor layer 59 constitutes the TFT's gate electrode G, while a part of the pixel electrode 56 that overlaps part of the semiconductor layer 59 constitutes the TFT's drain electrode D.
The color filter substrate CF has a configuration such that a color filter layer 61 of red, green, or blue, an overcoat layer 62, and an alignment layer 63 are provided on a surface of a second transparent substrate 60. To form the IPS mode liquid crystal display panel 50, the array substrate AR and color filter substrate CF are positioned opposing each other so that the pixel electrode 56 and common electrode 55 on the array substrate AR and the color filter layer 61 on the color filter substrate CF face each other, a liquid crystal LC is sealed therebetween, and polarizing plates 64 and 65 are deposed on the outer side of the substrates AR and CF, respectively, so that their polarization directions are orthogonal to each other.
In this IPS mode liquid crystal display panel 50, when an electric field is formed between the pixel electrode 56 and common electrode 55, the liquid crystals, which are aligned horizontally, will gyrate horizontally as shown in FIG. 9. By means of this it is possible to control the amount of incident light from the backlight that is transmitted. This IPS mode liquid crystal display panel 50 has the advantages of a wide viewing angle and high contrast, but also has the problems of low aperture ratio and transmittance because the common electrode 55 is formed from the same metallic material as the common wires 53 or scan lines 52, as well as the problem of color variation depending on the viewing angle.
FFS mode liquid crystal display panels that may be called “oblique field mode” (see JP-A-2002-14363, JP-A-2002-244158, JP-A-2003-195352, and JP-A-2005-309052) have been developed in order to resolve the problems of low aperture ratio and transmittance in IPS mode liquid crystal display panels. The operating principles of such an FFS mode liquid crystal display panel will now be described using FIGS. 10 and 11. FIG. 10 is a schematic plan view of a single pixel portion of an FFS mode liquid crystal display panel. FIG. 11 is a cross-sectional view along line XI-XI in FIG. 10.
This FFS mode liquid crystal display panel 70A has an array substrate AR and a color filter substrate CF. The array substrate AR has a plurality of scan lines 72 and common wires 73 provided in parallel with one another on a surface of a first transparent substrate 71, and a plurality of signal lines 74 provided in the direction orthogonal to these scan lines 72 and common wires 73. A common electrode 75 coupled to the common wires 73 and formed from indium tin oxide (ITO) or a like transparent material is provided so as to cover each space delimited by the scan lines 72 and signal lines 74. Over a surface of the common electrode 75 there are provided therebetween, with an insulator 76 interposed, a pixel electrode 78A constituted of ITO or a like transparent material in which a plurality of stripe-like slits 77A are formed. The surfaces of the pixel electrode 78A and the slits 77A therein are covered by an alignment layer 80.
Close to the positions where the scan lines 72 and signal lines 74 intersect there are formed TFTs that serve as switching elements. For each TFT, a semiconductor layer 79 is laid on a surface of the scan line 72, and a portion is extended from the signal line 74 so as to cover part of the surface of the semiconductor layer 79 and constitute the TFT's source electrode S; a scan line portion below the semiconductor layer 79 constitutes the TFT's gate electrode G, while a part of the pixel electrode 78A that overlaps part of the semiconductor layer 79 constitutes the TFT's drain electrode D.
The color filter substrate CF has a configuration such that a color filter layer 83 of red, green, or blue, an overcoat layer 84, and an alignment layer 85 are provided on a surface of a second transparent substrate 82. To form the FFS mode liquid crystal display panel 70A, the array substrate AR and color filter substrate CF are positioned opposing each other so that the pixel electrode 78A and common electrode 75 on the array substrate AR and the color filter layer 83 on the color filter substrate CF face each other, liquid crystal LC is sealed therebetween, and polarizing plates 86 and 87 are deposed on the outer side of the substrates AR and CF, respectively, in such a manner that their polarization directions are orthogonal to each other.
In this FFS mode liquid crystal display panel 70A, when an electric field is formed between the pixel electrode 78A and common electrode 75, the field is oriented toward the common electrode 75 at both sides of the pixel electrode 78A, as shown in FIG. 11. Consequently, not only does the liquid crystal present at the slits 77A move, but so does the liquid crystal present over the pixel electrode 78A. As a result, the FFS mode liquid crystal display panel 70A has the features of having an even wider viewing angle and higher contrast than the IPS mode liquid crystal display panel 50, and moreover an ability to provide bright displays thanks to possessing high transmittance. In addition, the FFS mode liquid crystal display panel 70A has a greater overlap area, viewed from above, between the pixel electrode 78A and common electrode 75 than the IPS mode liquid crystal display panel 50 has, and, as a collateral effect thereof, a larger holding capacity and hence the advantage that no auxiliary capacity line needs to be specially provided.
In an FFS mode liquid crystal display panel, similarly to the IPS mode liquid crystal display panel disclosed in JP-A-10-319371, it is preferable for the sake of the display characteristics that the rubbing direction should be orthogonal to the signal lines, and the pixel electrodes be provided at a slight inclined angle relative to the rubbing direction. Accordingly, a structure may be adopted whereby stripe-like slits 77B provided in a pixel electrode 78B are inclined relative to the scan lines 72 as in the FFS mode liquid crystal display panel 70B shown in FIG. 12. Similarly, in order to widen an aperture and provide brighter displays, an open end 77C′ is provided to one end of each slit 77C provided in a pixel electrode 78C, as in an FFS mode liquid crystal display panel 70C shown in FIGS. 13A and 13B. FIG. 13B is a cross-sectional view with the color filter substrate included along line XIIIB-XIIIB in FIG. 13A.
In order to eliminate color variation depending on the viewing angle, stripe-like slits 77D provided in a pixel electrode 78D may be arranged in two mutually inclined sets, one above the other, thus producing dual domains, as in an FFS mode liquid crystal display panel 70D shown in FIG. 14. Furthermore, the signal lines 74 may be provided in a crank-shape in a direction orthogonal to the scan lines 72, and a plurality of common electrodes 75E and pixel electrodes 78E be arranged in a delta layout, so that the signal lines 74 will not form straight lines, and the device will be well suited for image displays, as in an FFS mode liquid crystal display panel 70E shown in FIG. 15.
The FFS mode liquid crystal display panels 70B to 70D shown in FIGS. 12 to 14 differ from the FFS mode liquid crystal display panel 70A shown in FIG. 10 only in that the slits 77B to 77D provided in their pixel electrodes 78B to 78D are inclined and have different shapes. Moreover, the FFS mode liquid crystal display panel 70E shown in FIG. 15 differs from the FFS mode liquid crystal display panel 70A shown in FIG. 10 only in that slits 77E provided in its pixel electrodes 78E are inclined and that its a plurality of common electrodes 75E and pixel electrodes 78E are arranged in a delta layout. In the below, component elements that have identical structure to those in the FFS mode liquid crystal display panel 70A shown in FIG. 10 are assigned the identical reference numerals and detailed descriptions thereof are omitted.
While the common wires are provided in parallel with the scan lines for individual pixels in the FFS mode liquid crystal display panels 70B to 70E shown in FIGS. 12 to 15, the common wires can be provided anywhere between adjacent scan lines, and therefore they are not specifically shown. While the slits in pixel electrodes are provided in parallel with one another in a transverse direction of the scan lines in the FFS mode liquid crystal display panels 70A to 70E shown in FIGS. 10 to 15, they may be provided in parallel with one another in a longitudinal direction of the scan lines (not shown).
Thus, FFS mode liquid crystal display panels have the features of having an even wider viewing angle and higher contrast than IPS mode liquid crystal display panels, and moreover of being able to provide bright displays thanks to possessing high transmittance. Furthermore they can be driven with low voltage, and what is more, have a larger holding capacity generated as collateral effect, which means that they yield good display quality without special provision of auxiliary capacity lines.
There is formed in such an FFS mode liquid crystal display panel a transverse electric field primarily in a direction substantially orthogonal to the longer sides of the slits. When the panel is driven to transmit light, the slits in the pixel electrodes and pixel electrode portions laid in parallel with the slits transmit light in lines in such a manner that both the slits and pixel electrode portions appear to emit light in lines. On the shorter sides of the slits where their ends in the longitudinal direction are closed, a transverse electric field primarily in a direction substantially orthogonal to the shorter sides is formed. Consequently, the two-direction transverse electric fields cause a reverse twist in part of the liquid crystal molecules, whereby transmitted light cannot be accurately controlled and thus brightness is lowered.
In the FFS mode liquid crystal display panel 70C shown in FIG. 13, each slit 77C provided in the pixel electrode 78C has the open end 77C′ on one side, thereby making the liquid crystal element twist with an electric field normally formed in a portion before the open end 77C′ of each slit, which has a fringe effect. Consequently, a display opening area in this display panel is wider by a width M than an FFS mode liquid crystal display panel having slits whose both ends in the longitudinal direction are closed. As a result, an FFS mode liquid crystal display panel is obtained that provides brighter displays.
This arrangement of the slits in pixel electrodes extending in a direction crossing the signal lines, however, sometimes causes color mixture in the FFS mode liquid crystal display panel 70C having the slits whose ends are open on one side. Upon conducting a series of various investigations into the causes of the occurrence of the color mixture in such an FFS mode liquid crystal display panel having slits whose ends are open on one side, the present inventors found that it was due to causes described below.
As shown in FIG. 13B, which is a sectional view also showing the color filter substrate along line XIIIB-XIIIB in FIG. 13A, a centerline 91 along the signal line 74 of the pixel electrode 78C in each pixel is physically located at the center in the width direction of the pixel electrode 78C. In contrast, the display width L2 extends from one (closed) end of each slit 77C in the pixel electrode 78C to another (open) end 77C′ plus the width M as shown in FIG. 13B. Therefore, a display centerline 92 of the display region along the signal line 74 is located nearer to the open end 77C′ of each slit 77C. Since the right and left ends of the color filter coincident with centers of the signal lines 74 in the related-art arrangement, as viewed from above, Rm is larger than Lm (Rm>Lm) where Rm is a distance from the left end of the filter to a median position between adjacent display regions and Lm is a distance from the right end of the filter to that median position.
A distance ΔL from the centerline 91 of the pixel electrode 78C and the display centerline 92 is determined by the formula below:ΔL=(L1−L2)/2where L1 is the width of the pixel electrode 78C. The distance ΔL varies depending on manufacturing devices and other factors, and generally ranges from about 1 to 5 μm.
This causes a problem when the array substrate AR and color filter substrate CF are combined. The display region is located nearer to the edge of the pixel electrode 78C on the open end 77C′ side of each slit 77C, giving a low tolerance Lm for misalignment. In contrast, the display region is located away from the edge of the pixel electrode 78C on the closed end side of each slit 77C, giving a high tolerance Rm for misalignment. Consequently, supposing that the open end 77C′ of each slit 77C is on the right, moderate misalignment of the color filter substrate CF to the right will cause no change in displayed colors, but a bit of misalignment of the substrate CF to the left will result in color mixture because the substrate overlaps an adjacent display region. While a black matrix can be interposed between adjacent color filters in the color filter substrate CF to prevent color mixture, if an attempt is made to form the black matrix to prevent color mixture in consideration of the potential for misalignment of the substrate, the black matrix, which does not transmit any light, becomes so thick that transmittance will be compromised.